[Boatanchors] Advice Request on Heathkit HP-20 Power Supply

[D C _Mac_ Macdonald]

Thanks to Rodger, Glen, and Bob for your replies. 
 
I am using a Fluke 77 for my measurements.  The fact that my 
resistance measurements from each filament winding to ground 
show infinite resistance would seem to me to indicate that 
this IS capacitive coupling. I haven't had a good analog 
meter for years. 



There are NO bypass capacitors to chassis on the filaments. 
 
I'm now thinking that there isn't a real problem here. 
 
I have been looking for this HP-20 supply for over three years 
to pair up with the HX-20 and HR-20 SSB mobile gear I bought 
on the bay probably at least that far in the past! 
 
* * * * * * * * * * * 
* 73 - Mac, K2GKK/5 * 
* (Since 30 Nov 53) * 
* k2gkk at hotmail.com * 
* Oklahoma City, OK * 
* USAF & FAA (Ret.) * 
* * * * * * * * * * * 
 
 
 
 
 
> Date: Tue, 22 Jan 2013 19:22:36 -0600
> Subject: re: [Boatanchors] Advice Request on Heathkit HP-20 Power Supply
> From: WQ9E at btsnetworks.net
> To: k2gkk at hotmail.com
> 
> Mac,
> 
> I wonder how much of this is from capacitive coupling to ground in the transformer, if you are using a modern DMM it doesn't take much capacitive coupling to provide a high reading given the high meter input impedance. Simpson made a specialized leakage tester that incorporated a specified resistance to ground and provided a range of acceptable readings. I cannot recall the model number but I have read the manual and it provides sufficient information to duplicate the resistor/capacitor network to assess leakage. Of course first you might try the measurement with a classic older VOM like the Simpson 260 which presents a 5,000 ohms per volt impedance and see how much the measurement changes.
 
 73, Rodger WQ9E
 
  
 
 
 
Date: Tue, 22 Jan 2013 17:20:14 -0800
From: gzook at yahoo.com
Subject: Re: [Boatanchors] Advice Request on Heathkit HP-20 Power Supply
To: k2gkk at hotmail.com; boatanchors at mailman.qth.net
 



Are there bypass capacitors from each side of the filament windings to ground?  If so, there is a very good chance that what you are seeing is just like the old bypass capacitors from each side of the AC line to ground when the chassis is not grounded. 
 

In most radios, one side of the heater ("filament") voltage is connected to the chassis and that should eliminate this voltage problem.  You can remove the bypass capacitors and that shouldn't hurt anything. 
 

Also, installing a 3-wire cord should definitely be done for safety reasons.  Connect the green wire to the chassis, connect the black wire so that it goes through the fuse and then to the power switch.  The white wire goes directly to the "other" side of the primary winding. 

 
Glen, K9STH 
 

Website: http://k9sth.com 
 

  
  
  
  

From: macklinbob at gmail.com
To: k2gkk at hotmail.com
Subject: Re: [Boatanchors] Advice Request on Heathkit HP-20 Power Supply
Date: Tue, 22 Jan 2013 17:05:49 -0800

The filament supply in the HP-20 is supposed to be floating. The only 
chassis connection is the line filter.

It would not hurt to install a 3 wire powercord.
 
With power removed ohm the filament supply to the chassis.
 
Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark" 
 
 
 
 
 
----- Original Message ----- 
From: "D C _Mac_ Macdonald" <k2gkk at hotmail.com>
To: <boatanchors at mailman.qth.net>
Sent: Tuesday, January 22, 2013 4:46 PM
Subject: [Boatanchors] Advice Request on Heathkit HP-20 Power Supply
 
 I just bought a Heath HP-20 power supply on the bay.
It arrived today and is in beautiful physical condition.

I opened up the bottom and found a lot of stuff that
was NOT in the Heathkit manual.

While I didn't try to draw a schematic for it, what
was in the manual was mostly correct for the high and
low B+ and C- supplies.

The filament supply was something else! It appears
that a prior owner tried to rectify the filament supply
to provide DC to "something."

I removed all of that stuff and rewired the HP-20 to
original wiring status.

Next came a test of the outputs (no load). Both B+
and the C- bias supplies had reasonable voltages.

The rewiring of the filament section gave me a bit over
14 VAC, no load. Again, reasonable without load.

Then I decided to check the filament windings against
ground. OH, OH ! ! About 48 VAC from one side and
about 36 VAC from the other side to ground! There is
NO wiring from either side to ground.

I'm guessing (never ran into this in 59 years as a ham)
that the power transformer is leaky. The HP-20 has
had a two-wire cord installed; the known dangerous
in-line fuse plug is gone.

What are the dangers? Could installing a three-wire cord
help out here? I sure don't want to risk electrocution!

* * * * * * * * * * *
* 73 - Mac, K2GKK/5 *
* (Since 30 Nov 53) *
* k2gkk at hotmail.com *
* Oklahoma City, OK *
* USAF & FAA (Ret.) *
* * * * * * * * * * *